Lipofuscin is a pale yellow-brown pigment, made of free-radical-damaged protein and fat, which stains red with Sudan black and is PAS-positive. Granules of lipofuscin granules accumulate in the cytoplasm of muscle and nerve cells, normally in lysosomes, acting as a marker of wear and tear. They concentrate beneath the skin (forming so-called age spots or liver spots), in the muscles including the heart, and in the liver and other vital organs.
Lipofuscin is particularly troublesome in brain tissue where it accumulates over a lifetime. In the brain, when the lipofuscin finally reaches a critical level in a neuron, the neuron dies. Lipofuscin builds up normally with age or pathologically in certain conditions such as Alzheimer's disease, Parkinson's disease, and certain lysosomal diseases.